1. Field of the Invention
The present invention relates to a zoom lens system, and more specifically, to a zoom lens system having a large zoom ratio and applicable to a small-sized camera such as a video camera.
2. Description of the Prior Art
In recent years, camera bodies of cameras such as video cameras have remarkably been reduced in weight and volume compared to conventional ones owing to packaged electronic parts and improvement in integration rate thereof. The reduction of the prices and costs of camera bodies have also been remarkable.
Under such circumstances, the absolute values of the weight, volume and cost of the lens system with respect to the entire camera system have gradually been improved. However, since the relative values thereof with respect to the entire camera body are increasing year by year, a demand for a compacter, lower-cost lens system is increasing.
On the other hand, higher functions, such as a large aperture ratio for compensating for insufficient illumination caused due to the reduction in size of the imaging device and a higher performance for coping with higher pixels and higher resolution, have been demanded of lens systems.
Assuming here that the zoom ratio is approximately 8.times. which is currently popular in the field of the video camera, a large-aperture-ratio zoom lens system will be discussed which has an F number of approximately F 1.8. Many lens systems having such a specification have been disclosed in the form of zoom lens systems having four or five lens units. However, most of them, which have approximately 13 to 15 lens elements, no longer meet the current demand both in cost and size.
Recently, there has been a movement to reduce the number of lens elements by using aspherical surfaces. For example, a zoom lens system disclosed in Japanese Laid-open Patent Application S57-27219 is, although it is not an 8.times. magnification zoom lens system, a three-unit lens system of a positive, negative, positive configuration. The first lens unit and the second lens unit are moved along the optical axis as an image point position compensating unit (compensator) and as a zooming unit (variator), respectively, and by using one aspherical surface in each lens unit, a zoom lens system having an F number of F 1.6 is realized with 12 lens elements.
In this lens system, however, the zoom structure, lens configuration and lens arrangement are not effective, and the number of lens elements is not sufficiently reduced in view of its specification. Further, it is impossible for this zoom lens system to cope with a high zoom ratio of approximately 6.times.. This is because the zoom lens system has the following defect other than the above-mentioned inappropriate lens configuration and lens arrangement. That is, since the third lens unit is not moved during zooming, the first lens unit is inevitably moved as a compensator lens unit; in order to realize a high magnification of approximately 6.times. with this arrangement, the diameter of the first lens element (front lens element) considerably increases at a WIDE limit (shortest focal length condition) and in a middle range (middle focal length condition) compared to four- or five-unit zoom lens systems, so that the weight considerably increases.
On the contrary, Japanese Laid-open Patent Application No. S61-110112 and Japanese Laid-open Patent Application No. S60-107013 disclose four-unit zoom lens systems where considerably effective lens configuration, lens arrangement and aspherical surface arrangement are realized and where much fewer lens elements are used.
Japanese Laid-open Patent Application No. S61-110112 discloses a four-unit zoom lens system of a positive, negative, negative, positive configuration. By simplifying each lens unit and by effectively using four aspherical surfaces, a 6.times. magnification zoom lens system is realized with only eight lens elements. However, although the lens arrangement is excellent, the aberration performance is rather inferior. It is consequently difficult to realize the currently-demanded performance.
Japanese Laid-open Patent Application No. S60-107013 shows a schematic view of a four-unit zoom lens system of a positive, negative, positive, positive configuration including eight lens elements. Since no numerical data are shown, it is impossible to determine its performance and size. Moreover, since it is a 4.times. magnification zoom lens system having an F number of F2.0, it is impossible to apply it to the presently-discussed case (that is, 8.times. magnification zoom lens system having an F number of F 1.8).
Moreover, although the zoom ratio is low, Japanese Laid-open Patent Application No. S63-304218, Japanese Laid-open Patent Application No. S64-44907 and Japanese Laid-open Patent Application No. H1-223408 propose zoom lens systems where the number of lens elements is remarkably reduced by using aspherical surfaces. They are three-unit zoom lens systems of a positive, negative, positive configuration where a second lens unit includes one lens element and a first lens unit includes one or two lens elements.
In these zoom lens systems, since the second lens unit which serves as a zooming lens unit and largely moves along the optical axis includes one single negative lens element and chromatic aberration is not corrected in the second lens unit, variation in chromatic aberration due to zooming is large. It is consequently impossible to guarantee satisfactory performance when the lens systems are provided with a high zoom ratio. In fact, these prior arts realize zoom ratios of only from 2 to 3.times. and F numbers of only 2 to 4.
The above-mentioned variation in chromatic aberration cannot be improved even if many aspherical surfaces are used. In these zoom lens systems, if the currently-demanded performance (including chromatic aberration)is realized, a zoom ratio of only at most 3.times. can be realized, and it is consequently impossible to realize a zoom ratio of 8.times..
Further, Japanese Laid-open Patent Application No. S64-91110 and Japanese Laid-open Patent Application No. H1-185608 propose novel zoom lens systems. A zoom lens system of Japanese Laid-open Patent Application No. S64-91110 realizes a zoom ratio of 3.times., in spite of having a lens configuration nearly the same as that of a three-unit lens system, by causing it to work substantially as a four-unit lens system by separating a portion corresponding to a second lens unit into a negative unit consisting of two negative lens elements and a positive unit consisting of one positive lens element and by reducing the number of lens elements up to 8 to 11 which numbers are common for three-unit lens systems. Zooming is performed by independently moving the above-mentioned negative and positive units. However, the essential weak point of this four-unit zoom lens is that, since chromatic aberration correction is not completed in each of the second and third lens units which independently move, it is impossible to sufficiently restrain a variation in chromatic aberration at a high zoom ratio due to a variation in relative position of each lens unit during zooming. In this example, restriction of the variation in chromatic aberration is managed by contriving an optimal power arrangement; however, it is quite difficult to realize 6.times. magnification in this zoom lens system.
Japanese Laid-open Patent Application No. H1-185608 realizes a 6.times. magnification zoom lens system by reducing the number of lens elements of the zoom lens system of Japanese Laid-open Patent Application No. S64-91110 by using aspherical surfaces. That is, this is a zoom lens system where the second lens unit of the zoom lens system of Japanese Laid-open Patent Application No. S64-91110 consists of one single negative lens element, the third lens unit thereof consists of one single positive lens element and the fourth lens element thereof is also simplified. In this lens system, however, since the above-mentioned variation in chromatic aberration is large, remaining chromatic aberration is still large although the power arrangement is carefully chosen, and consequently, it is difficult to realize the currently-demanded performance. Further, since chromatic aberration correction is attached a considerable amount of importance to in choosing an optimal power arrangement, the movement amounts of the second and third lens units which are movable lens units are quite large. As a result, the total length of the zoom lens system increases. In addition, since the diameter of the front lens element which has a great influence on the weight of the entire lens system is quite large compared to existing typical ones of the same specification, this zoom lens system is inferior from the viewpoint of the realization of compactness. As described above, the zoom lens system proposed by Japanese Laid-open Patent Application No. H1-185608 does not meet the current needs in compactness and chromatic aberration performance, although it achieves an object of reducing the number of lens elements.
Further, Japanese Laid-open Patent Application No. H2-39011 discloses a four-unit zoom lens system having a positive, negative, positive, positive configuration where chromatic aberration can be restrained similarly to the zoom lens system of Japanese Laid-open Patent Application No. H1-185608. In this zoom lens system, three aspherical surfaces are employed, and a 6.times. magnification zoom lens system having an F number of F 1.4 is realized with eight lens elements. It appears that the possibility of realizing the currently-demanded performance is stronger in cost, performance and size than the above-mentioned prior arts. However, there sill remain some problems: since the diameter of the front lens element is not sufficiently small, the zoom lens system has no considerable advantage in weight over conventional ones; and since coma in a sagittal direction which hardly appears in a graphic representation of aberrations is very large, deterioration of off-axial performance is large.
Japanese Laid-open Patent Application No. S54-30855, Japanese Laid-open Patent Application No. S54-80143 and Japanese Laid-open Patent Application No. H2-39116 propose zoom lens systems for use in single-lens reflex cameras and compact cameras. They are three-unit zoom lens systems having a positive, negative, positive configuration where high zoom ratios are realized by reducing the number of lens elements by moving each lens unit. The zoom ratio and the number of lens elements of the three prior arts are respectively 2.4.times. magnification and 10 lens elements, 3.times. magnification and 11 lens elements, and 3.times. magnification and 12 lens elements. The zoom ratios are insufficient, and since the second and third lens units are not sufficiently simplified, the costs are not satisfactorily reduced.